Kids in England see us in New Zealand as "down under". To them we are upside
down and ought to fall off the Earth! We don't because everything on Earth is
attracted towards the centre of the Earth by gravity. All massive things like
the Sun and the Moon pull at us by gravity but the pull is very weak if the
things are very distant. If the Moon or Sun (or both) happened to be right above
you, you would be a tiny bit lighter because of the upward pull of the Moon (or
Sun). So you might expect the sea around you to be a bit lighter too and so
flow in from elsewhere to make a high tide a metre or higher than if the Moon
were low on the horizon instead of right above us.

Happy with that? If you are very observant, you'll say: "Hey! Wait on!
There are TWO high tides a day as the Earth rotates, not just one! That
explanation can't be the whole story!" And you would be right!

The Moon and the Earth are massive solid things which pull on one
another from and at their centres. The sea on the side of the Earth
nearest the Moon at any time feels a stronger pull by the Moon
than the Earth feels because that bit of the sea is closer to the moon.
So there is a high tide there, sucked up a little by the Moon. Aha,
but what about the sea on the opposite side of the Earth, furthest
from the Moon? THERE the sea feels a WEAKER pull by the
Moon than the Earth feels because that bit of the sea is further
from the Moon than the Earth is, so it also feels a little lighter
and makes a high tide there too.

That explains why there are two high and two low tides each day
instead of one, and why the high tides are higher and the low tides
lower when the Sun and Moon pull together at Full Moon and
at New Moon.